Method of cleaning surfaces with water and steam

ABSTRACT

A method and device for cleaning contaminated surfaces with both water and steam. A water film is applied to the contaminated surface and steam sprayed into the water film while water is continued to be applied to the water film. In this way the steam is directed into the water film and through the water film onto the contaminated surface, whereby contaminates in the contaminated surface are removed.

FIELD OF THE INVENTION

The present invention relates to a method for super-cleaning surfaces,and to a device in accordance with the preamble of for executing thismethod.

BACKGROUND OF THE INVENTION

Conventional cleaning methods are based in immersion of the surface ofthe article or substrate to be cleaned into a wet bath, filled withdetergent for cleaning, and, for instance reinforced by ultrasound, orby immersion of the substrate in vapor atmospheres, or by mechanicaltreatment of the surface, for instance by brushing, spraying of water ordetergent at high pressure from spray nozzles, or by vibrationalexcitation vibration (megasonic cleaning).

Particularly in chip manufacture, planarization is a process step thatis gaining ever greater significance as the miniaturization ofstructures progresses. The structured surfaces of a substrate (forinstance of a silicon wafer) are ground or lapped flat by achemical-mechanical polishing step (or CMP processing step), using finegrinding agents (slurry). These grinding agents have an extremely smallparticle size, with the attendant disadvantage that after the grindingprocess, individual grinding particles still adhere to the machinedsurface and are partly embedded in the surface. After the grindingprocess, the surface must be freed completely of grinding agents andother contaminants so that the substrate can be further machined.

As already noted above, by means of brushes, water and by spraying waterat high pressures of up to 50 bar, an attempt is made to remove thegrinding agent residues. The brushes have the substantial disadvantagethat they must be constantly wetted with water and that they graduallyfill up with the removed grinding agents. The brushes must therefore bereplaced regularly, which in turn has the disadvantage that byintervention with the cleaning station on replacement of the brushes,the cleaning station becomes contaminated. Mechanically treating thesurface to be cleaned with the brushes also involves the danger that thesurface will become damaged and in particular scratched. As a rule, thecleaning process with the brushes is followed by a further cleaningprocess in which water is sprayed at high pressure. If good results areto be obtained, the surface to be cleaned must be exposed to the sprayfor a certain period of time and stopped repeatedly, which makes for along cleaning time.

The disadvantages of the conventional method will be listed again below.Along with major brush wear and hence high costs for the brushes,frequent brush changing involves a necessary intervention into themachine and hence a machine shutdown. Because the brushes become filledwith particles, damage to the surface by scratching must be feared, andthe fact that the brushes (the brush material is PFA sponge) are keptpermanently wet means major consumption of deionized water. Moreover,designing the cleaning chamber with brush cleaning for cleaning bothsides of the substrate is complicated and expensive. The contactpressure of the sponge brushes cannot be adjusted replicably. Inhigh-pressure cleaning, there is the risk of electrostatic charging ofthe substrate, since nonconductive deionized water is used. Withmegasonic cleaning, the surface cannot be cleaned completely, especiallyafter grinding processes.

From European Patent Disclosure EP0381435A2, a method and a device forwashing printed circuit boards is known. Steam is sprayed at the boardsat a temperature and a pressure at which the steam undergoes no changeof phase.

Japanese reference JP-A 703 7866 discloses a device with which anarticle is exposed to an atmosphere of steam enriched with NO₂.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a method and/or adevice available, by means of which contaminated surfaces can be cleanedmore simply, at lower cost and more rapidly, as well as with increasedprotection from damage.

This object is attained in accordance with the present invention in thatin the method aspect at least one steam nozzle is brought into theimmediate vicinity of the surface, and in that the steam is directed ina targeted way to the surface to be cleaned via the nozzle, and thesurface to be cleaned is moistened with water simultaneously with orprior to the spraying with steam.

The device aspect in accordance with the present invention has theessential advantage that the surface to be cleaned is not mechanicallytreated, and the adhering particles of the grinding paste which are tobe removed are on the one hand loosened by means of the heat energyintroduced via the steam. Because of the different heat absorption ofthe substrate and the particles to be removed, and because of tensioncaused by different coefficients of expansion, the connection betweenthe particles and the substrate is weakened, as a result of which theparticles are loosened from the surface to be cleaned. If wet steam isused in place of superheated steam, the particles are additionallyloosened by the transfer of pulse forces. In the process the kineticenergy (ultrasound energy) when the liquid droplets impinge is utilized.A further advantage resides in that in the course of introducing thesteam into a water film present on the surface, the water partiallyturns into steam locally, and the vapor bubbles either collapse againimmediately afterwards or burst on the surface of the water film.

The method in accordance with the present invention can be employed notonly with flat or plane surfaces, but also with deep structures, such asoccur in the production of micro-mechanical elements, for example. Theremoval of the loosened particles can take place either by means of thesteam directed on the surface, by the condensate, and/or by additionallysprayed-on water. This is of advantage in particular with very thinsubstrates, since only little condensation takes place there.

In a further development it is provided that the surface can be dried atthe end, for example by the introduction of foreign gas, such asnitrogen or the like, or by employing hot steam. Spots left after dryingare thus averted. It is considered to be particularly important that theremoval of the particles takes place by means of the action of pulses(kinetic energy), by different heat expansion (thermal energy) and bymeans of the action of pulses (water turning into steam and bursting ofthe gas bubbles). In this connection, blowing of steam into a water filmis preferred.

Advantages are seen in the employment of deionized water, to whichalcohols, detergents, solvents or the like can be added. Foreign gases,for example nitrogen or the like, can be added to the steam. Oneparticular area of application is considered to be the cleaning whichfollows the CMP process, but others are also in connection with themanufacture of LCD products, production of hard disks, and inmicro-mechanics or other processes requiring super-clean surfaces.

A transport device is provided in accordance with the present invention,by means of which objects, for example substrates whose surface needs tobe cleaned, are rotated and/or conveyed. In the case of silicon wafersthe objects are, for example, flat disks, which are transported withoutcontact through the cleaning station in a continuous running process. Inthe process both the top and the bottom are cleaned. The two sides canbe cleaned simultaneously or one after the other. In the process theobject preferably rotates on a steam and/or water cushion and ismaintained contactless in this way. The position is fixed by pins orrollers acting on the edge of the disk. The substrate is caused torotate and/or is transported through the cleaning station byappropriately disposed nozzles on the transport device, which aredirected on the underside of the disks. The oblique impact angle of thefluids leaving the nozzles has been selected such, that disk is moved inany case, possibly even cleaned in addition. The impacting fluid istherefore used as a transport medium, cleaning medium and removal mediumfor loosened particles. The runoff of the fluids is made easier, forexample by transverse grooves or obliquely extending grooves in thetransport device.

A drying section, in which the substrate is dried, can follow thesection of the cleaning station where cleaning takes place. In theprocess the substrate is preferably caused to rotate (1500 rpm), bymeans of which the adhering water film is thrown off, drying can also beaided by hot steam. The individual substrates can be inserted intomagazines or cassettes at the outlet from the cleaning station.

The advantages of the method in accordance with the present inventionand the device in accordance with the present invention are essentiallythe following: improved cleaning results at higher eater temperatures;shorter process times and attendant higher throughput; contactlesscleaning and therefore no mechanical stress of the substrate surface; nodisposable articles such as brushes, etc., and therefore reducedoperating costs; no interference with the cleaning chamber and thereforeno down times and no danger of contamination; no cleaning of the processchamber is required, since it is automatically also cleaned by thesteam; smaller size of the installation and lesser need for clean rooms;simpler structure of the process chamber, reproducible cleaning resultsin contrast to the customary processes, in which the brushes are usedup; easier integration in a cluster with other installations; andenvironmentally friendly cleaning, since in contrast to cleaning by theCMP process no solvents are involved.

Further advantages, characteristics and details of the present inventionwill become apparent from the claims and the ensuing description, inwhich especially preferred exemplary embodiments are shown in detail inconjunction with the drawing. the description and shown in a drawing mayeach be essential to the invention individually or in arbitrarycombination.

Shown in the drawing are:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, is a basic sketch of a device for generating steam;

FIGS. 2a-2c, a process sequence for coating and planarizing a siliconwafer;

FIG. 3, is a first embodiment of the cleaning device according to thepresent invention;

FIG, 4, is a second embodiment of the cleaning device according to thepresent invention;

FIG. 5, is a first embodiment of a transport device according to thepresent invention;

FIG. 6, is a second embodiment of a transport device according to thepresent invention;

FIG. 7, is a plan view on the transport device of FIG. 6 in thedirection of the arrow VII;

FIG. 8, is a third embodiment of the transport device according to thepresent invention;

FIG. 9, is a fourth embodiment of the transport device according to thepresent invention;

FIG. 10, is a fifth embodiment of the transport device according to thepresent invention;

FIG. 11, is a basic illustration of a cleaning station;

FIG. 12, is an illustration of a first method for cleaning contaminatedsurfaces;

FIG. 13, is an illustration of a second method for cleaning contaminatedsurfaces;

FIG. 14, is an illustration of a third method for cleaning contaminatedsurfaces;

FIG. 15, is a plan view of the illustration of FIG. 14 in the directionof the arrow XV.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a basic sketch is shown for preparing the steam to be used toclean contaminated surfaces. Reference numeral 1 indicates a spraynozzle from which steam 16 emerges in the form of superheated steam orwet steam. The nozzle 1 communicates via a line 17 with a steam boiler5. Provided in the line 17 in succession are a steam vale 4, aconnection 12' for introducing foreign gas, for instance for introducingnitrogen (N₂), a connection 11' for introducing foreign agents, such asalcohol or the like, a superfine filter 3, and a flow reducer 2, forinstance in the form of a throttle. Located in the steam boiler 5 isdeionized water 18, which is introduced wither via a direct water inlet8, for instance in non-continuous operation, or via an inflow line 19.For the continuous mode of operation, this inflow line 19 communicateswith a water connection 15 and with a water pressure pump 14 and a waterinlet valve 13. Located in the water bath of the steam boiler 5 is aheater 6, in particular an electric heater, by means of which the waterbath is heated via a heater regulator 7 with a temperature sensor 20.The steam boiler 5 is also provided with a connection 12 for introducingforeign gas and a connection 11 for introducing a foreign agent, as wellas a steam pressure switch 10 and an overpressure valve 9. In this way,both superheated steam and wet steam can be generated, the latter forinstance by supplying water via the connection 11'.

In wafer manufacture, the structure surface of a silicon wafer 21 iscoated with a planarizing layer, which for instance comprises SiO₂. Thestructure surface, which has been produced by exposure to light andensuing etching, is shown in FIG. 2a. FIG. 2b shows the surface coatedwith the planarizing layer 22; it can clearly be seen that the surfaceof this planarizing layer 22 is not plane. Via an ensuing grinding orlapping process (CMP process), the planarizing layer 22 is ground downfar enough that only the indentations 23 in the surface of the siliconwafer 21 are filled. In this operation, a very fine grinding agent(slurry) is used, but the individual slurry particles still stick to thesurface or are embedded in indentations of the surface 60 after thegrinding process is finished. The surface shown at 24 in FIG. 2c mustthen be decontaminated for further processes, or in other words freed ofany contamination whatever.

FIG. 3 shows a first exemplary embodiment of the cleaning deviceaccording to the present invention, which is embodied for instance forholding silicon wafers 21, which have a round disklike shape. Thesilicon wafer 21 or the substrate 21 is fastened in the region of itsedges in at least three diabolo-shaped drive rollers 25 which arerotatably supported in the direction of the arrow 26. In this way, thesubstrate 21 can be moved without damage to or contact with the surface24. The two surfaces 24 are sprayed with steam 16 via spray nozzles 1from above and below. The spray nozzle 1 can be moved, each via asuitable device, radially in the direction of the arrows 27 or in someother way between the center 28 and the edge of the surface 21, and theyare spaced apart from the surface 24 by approximately 1 mm. As a result,the entire surface 24 can be treated with steam 16.

The steam 16 emerging from the nozzle 1 strikes the surface 24, so thatthe contaminants adhering to the surface 24 or located in indentationsof it are loosened by the kinetic energy. If wet steam 16 is sprayed onthe surface, then the contaminants are loosened by the liquid dropletsstriking them or by the kinetic energy of these droplets. The watervapor also condenses on the surface and is partly re-evaporated by thesteam that follows it, with the vapor bubbles collapsing immediatelyafterward or bursting at the surface of the substrate. These impulsiveforces cause further loosening of the particles. Moreover, the substrate21 and the individual particles are heated to different extents, causingthermal strains that lead to still further loosening.

The advantage of this method according to the present invention residesin the very low consumption of media, and the effect of the pulses, heatand impulsive force bring about very effective cleaning. The cleaningforce effect is also readily controllable by suitable variation of thesteam pressure, quantity of incident steam, steam temperature, spacingbetween steam nozzle and substrate surface, and thickness of the waterfilm as well as by the metering of foreign agent or foreign gas. Thecleaning is effected by means of deionized water, so that no disposalproblems occur and the entire method is environmentally friendly. Thedrying can be done by blowing hot steam 16 or by supplying foreign gas.The pressure is regulated by the temperature of the water in thetemperature range from 100° C. to approximately 200° C., which isequivalent to a pressure range up to about 10 bar.

FIG. 4 shows another embodiment of the cleaning device according to thepresent invention, in which the substrate 21 is fastened in a rotarymount 29. The rotary mount 29 can also hold substrates 21 that are notcircular and engages them in the region of their edge. In thisembodiment, the upward-pointing surface 24 is cleaned by means of thenozzle 1.

FIG. 5 shows a first embodiment of a transport device 30 for moving thesubstrate 21 in the direction of the arrow 31. Top this end, thetransport device 30 has a steam conduit 32, which is provided on its topwith many steam nozzles 33. These steam nozzles 33 are inclined slightlyin the direction of the arrow 31, so that steam 16 emerging from thesteam conduit 32 forms a steam cushion 34 under the substrate 21 andtransports it in the direction of the arrow 31. In this way, thesubstrate 21 can be moved without being touched in a once-throughpassage through a cleaning station, with the substrate 21 floating on acushion 34 of steam and water. The lateral guidance is effected viaboundary strips, as shown in FIG. 7.

The transport device 30, in addition to transport portions shown in FIG.5 also has portions 36 in which the substrate 21 is rotated, which isshown in FIG. 6. Steam nozzles 33 are also provided in these portions36, but their exit direction is inclined substantially upward and at atangent around the center 28 and optionally extends in inclined fashionoutward, as shown in FIG. 6, in the direction of the edge of thesubstrate 21. In this way, the substrate is rotated about the center 28in the direction of the arrows 38. The positioning of the substrate 21is effected via boundary pins 39, which are disposed so as to belowerable in the portion 36. If the boundary pins 39 have been extended,as shown in FIGS. 6 and 7, then the substrate 21 is impeded from beingtransported further in the direction of the arrow 31 and is firmly heldin the portion 36. The boundary pins 39 may also themselves be rotatablysupported. In that case, the cleaning of the surface or surfaces 24 ofthe substrate 21 can take place in the portion 36.

FIG. 8 shows a modified embodiment of a transport portion 40 of atransport device 30; the substrate 21 is disposed substantially betweentwo steam conduits 32, and both surfaces are acted upon by steam 16emerging from steam nozzles 33. The substrate 21 is thus embeddedbetween two steam cushions 34 and is moved in the direction of the arrow31.

In FIG. 9, an alternative cleaning portion 41 of the transport device 30is shown, in which the substrate 21 is firmly held via the boundary pins39 and rotated by means of the steam nozzles 37, as described for FIGS.6 and 7. The steam nozzles 37, as also already shown in the exemplaryembodiment of FIGS. 6 and 7, may simultaneously act as spray nozzles 1,or in other words for cleaning the surfaces 24 of the substrate 21. Thedetached dirt particles and the incident water can be removed, as shownin FIG. 10, via conduits 42 that are provided in the lower steam conduit32. To that end, the conduits 32 and/or 42 are inclined.

FIG. 11 schematically shows one such once-through device, in which thesubstrate 21 is placed on the transport device 30 and transported in thedirection of the arrow 31. The portion 35 serves to transport thesubstrate 21, and the portion 36 serves to clean it. Reference numeral43 schematically indicates a spin dryer, in which the substrate 21 isset into rapid rotation (about 1500 rpm) in the direction of the arrow44. Via suitable manipulators 45, the substrate 21 is removed from thespin dryer 43 and placed for instance in cassettes or magazines (notshow) and prepared for further transportation in the direction of thearrow 31.

In FIG. 12, a cleaning method is shown in which the substrate 21 haswater 18 sprayed on it via a water lance 46, thus creating a water film47 on the surface 24 of the substrate 21. Via the water lance 46, thewater 18 is fed onto the surface 24 at the center 28, so that the water18 runs off in the direction of the arrows 48 in the form of a waterfilm 47. At the same time, the spray nozzle 1 sweeps over the surface 24in the direction of the arrow 27 and sprays steam 16 directly into thewater film 47. Vapor bubbles thus form in the water film 47, whicheither immediately collapse again or burst at the surface of the waterfilm 47. Along with the pulsing action of the water vapor 16, theparticles 49 also undergo the impulsive action of these vapor bubbles,and are loosened or detached in this way from the surface 24. They arethen carried away along with the outflowing water film 47.

In the embodiment of FIG. 13, the substrate 21 is inclined at an angle αand is transported in the direction of the arrow 31 via transportrollers 50, only two of which are shown. Once again, water 18 forforming a water film 47 is applied to the surface 24 of the substrate 21via the water lance 46. Via the spray nozzle 1, water vapor 16 is blowninto this water film 47, as in the exemplary embodiment of FIG. 12. Inthis process, the spray nozzle 1 is moved orthogonally to the plane ofthe drawing, or the spray nozzle 1 may be embodied as a wide-slitnozzle, as shown in FIG. 15.

In the exemplary embodiment of FIG. 14, the substrate 21 is disposedflat and is also transported via rollers 50 in the direction of thearrow 31. Once again, via the lance 46, water 18 for forming a waterfilm 47 is applied to the surface 24, and steam 16 is blown into thewater film 47 via the nozzle 1. The plan view shown in FIG. 15 showsthat both the water lance 46 and the nozzle 1 are embodied as wide-slitnozzles. This arrangement is excellently suited to the once-throughmethod, and the substrate 21 is handled gently.

In closing it should also be pointed out that this method and acorresponding device can be retrofitted easily in existing grindingsystems and can readily replace existing cleaning device.

I claim:
 1. A method for cleaning contaminated surfaces with water andsteam, comprising the steps of:applying a water film to the contaminatedsurface; and spraying steam into the applied water film while continuingto apply water to the water film, wherein the steam is applied throughat least one steam nozzle and the water is applied through a separatedevice, and wherein the steam is directed into the water film andthrough the water film onto the contaminated surface, whereincontaminates in the contaminated surface are removed.
 2. The method asdefined in claim 1, wherein the steam is selected from the groupconsisting of superheated steam and wet steam.
 3. The method as definedin claim 1, wherein the water film is comprised of deionized water. 4.The method as defined in claim 1, further comprising the step of:addinggas and/or liquid to the steam and the water film.
 5. The method asdefined in claim 1, wherein the contaminated surfaces each define acenter and an edge, and wherein the cleaning process takes place fromthe center of each surface toward their respective edges.
 6. The methodas defined in claim 1, further comprising the step of:rotating thecontaminated surface to be cleaned.
 7. The method as defined in claim 1,wherein the contaminated surface defines an edge, and wherein said steamand said water are applied orthogonally to the contaminated surface. 8.The method as defined in claim 1, further comprising the step of:dryingthe contaminated surface after cleaning by a spin dryer.
 9. The methodas defined in claim 1,wherein the contaminated surface moves in aonce-through passage through a cleaning station.
 10. The method asdefined in claim 1, wherein the steam is sprayed onto the contaminatedsurfaces at a pressure of up to 10 bar.
 11. The method as defined inclaim 1, wherein the cleaning process is employed in one or more of thefollowing: chemical-mechanical-polishing processes, mask reduction, filmproduction, liquid-crystal-display production, hard-disk manufacture,micro mechanics reading heads for hard disks.
 12. The method as definedin claim 1, wherein the contaminated surface defines an edge, andwherein said steam and said water are applied obliquely of the edge ofthe contaminated surface.
 13. The method as defined in claim 4, whereinthe gas is nitrogen, and the liquid is selected from the groupconsisting of alcohols and detergents.